CHEMISTRY & CHEMICAL TECHNOLOGY
Vol. 10, No. 4, 2016
Chemistry
Suellem Cordeiro, Leticia Pereira, Marina de O. Simoes
and Maria de Fatima Marques
SYNTHESIS AND EVALUATION OF NEW BIS(IMINO) PYRIDINE
BASED CATALYSTS FOR ETHYLENE POLYMERIZATION
Universidade Federal do Rio de Janeiro, Instituto de Macromoleculas Eloisa Mano
IMA-UFRJ, Cidade Universitaria. Av. Horacio Macedo, 2.030
Centro de Tecnologia, Bloco J. Rio de Janeiro, RJ. Brasil; [email protected]
Recieved: May 03, 2016 / Revised: May 18, 2016 / Accepted: July 16, 2016
 Cordeiro S., Pereira L., Simoes M., Marques M., 2016
Abstract. Three-component catalytic systems based on
2,6-bis(imino) pyridine iron(II) chloride were synthesized
from different ligands, which provided new alternative
catalysts for polymerization of ethylene. The synthesized
catalysts were characterized by Fourier transform infrared
spectroscopy (FTIR), and the lack of absorption bands
was observed in the region related to the carbonyl, as well
as the presence of bands in the region of imino groups
corresponding to C=N bonds. Coordination with Fe was
also carried out. The structure of the ligands and the new
catalysts were confirmed by the elemental analysis
(CHN), and 1H- and 13C-nuclear magnetic resonance
spectroscopy. In ethylene polymerization with methylaluminoxane as a cocatalyst, the activity of catalyst C1 was
high. Although this catalyst structure contains sterically
bulky ligands, the metal center was not sufficiently protected allowing transfer reactions, producing polyethylene
with a low molar mass and melting temperature.
Keywords: post-metallocene catalysts,
pyridine ligands, ethylene polymerization.
bis(imino)
1. Introduction
Great attention has been focused on the study of
catalysts, in particular single site systems such as postmetallocene catalysts, which has been developed over the
years and became capable of synthesizing polymers in
larger amounts, with high performance and unique
characteristics [1, 2]. Due to these studies, it is possible to
proceed further advances in the architecture of the
catalyst, which allows a wide range of variations in the
ligand [3, 4], which may lead to an overall control of the
microstructure of the polymer and, hence, the properties
of the final product.
The new family of late-metal catalysts was
independently discovered by Brookhart, Bennett and
Gibson, the five-coordinate 2,6-bis(arylimino)pyridyl
Fe(II) dihalide types, activated by MAO, are effective
catalysts for the conversion of ethylene either to highdensity polyethylene or to α-olefins with Schulz-Flory
distribution [5]. Remarkably, the productivities were as
high as those of most efficient metallocenes.
The advantages of these Fe catalysts over other
types of single-site and Ziegler-Natta catalysts for
ethylene homopolymerization (e.g., metallocenes and
constrained geometry complexes) are multiple, from the
easiness of synthesis and the use of low-cost metals to low
environmental impact. Another intriguing characteristic of
bis(organylimino)pyridyl Fe(II) precursor is provided by
the easy tuning of their polymerization activity by simple
modification of the ligand architecture [6, 7].
Surrah et al. [8] describe the synthesis of
complexes bearing ligands that contain bulky aromatic
terminals such as pyrenyl, 2-benzylphenyl, and naphthyl
where Fe(II) catalyst containing the ligand 2-benzylphenyl
could convert ethylene to a linear polymer, while ligands
pyrenyl and naphthyl produced branched polymers. In
fact, it has been shown that the size, nature and
regiochemistry of the substituents in the iminoaryl groups
are extremely important in controlling the polymerization
and oligomerization of ethylene [9, 10]. Iron complexes
bearing small substituents at the 2-positions of the aryl
groups proved to be excellent catalyst precursors for
oligomerization of ethylene. These α-olefins are useful
and industrially highly desired, such as for
copolymerization reactions with ethylene to give linear
low-density polyethylene (LLDPE) [11-13].
Due to its low cost and ready availability for using
iron as a polymerization active centre, the objective of the
414
Suellem Cordeiro et al.
present study was to develop new homogeneous catalytic
systems comprehending iron complexes with different
compounds based on bis(imino) pyridine ligands,
functionalized for using in the future to polymerize
ethylene by heterogeneous systems.
2. Experimental
All manipulations of air- and/or moisture-sensitive
compounds were routinely performed under an inert
atmosphere using a glove box and/or standard Schlenk
techniques.
2.1. Materials
Methylaluminoxane (MAO) was purchased from
Crompton Corporation, Germany, as 1.9 M toluene
solution. Toluene (Ipiranga Petrochemical, commercial
purity) was distilled over sodium/benzophenone ketyl
prior to use. n-Butanol (P.A.) was dried over calcium
hydride. Ethylene was supplied by White Martins Gases
Industrial, purity 99.9 %, and eluted in columns 3A
molecular sieve and copper catalyst. All other starting
materials were commercially available and were used
without further purification.
For the production of complexes we used the
following reagents: 2,6-diacetylpyridine, bis(4-aminophenyl)-1,4-diisopropylbenzene, aniline, 2,6-diisopropylaniline, p-phenylenediamine. They were purchased from
Sigma-Aldrich.
2.2. Synthesis of Ligands
Four different ligands (L1-L4) were prepared by
the following methods.
2.2.1. Synthesis of L1 and L2 ligands
To a solution of 2,6-diacetylpyridine (18.4 mmol)
in an absolute ethanol (30 ml), bis(4-aminophenyl)-1,4diisopropylbenzene (L1) or aniline (L2) (53 mmol) was
added. After the addition of a few drops of glacial acetic
acid, the solution was refluxed for 40 h. Upon cooling to
room temperature, the product was crystallized from
ethanol. After filtration, the yellow solid (both L1 and L2)
was washed with cold ethanol and dried under vacuum
(5 Pa). The yield was 5.75 g (82 %).
This procedure was carried out according to the
analogous method used by Britovsek et al. [14].
2.2.2. Synthesis of L3 and L4 ligands
To obtain the catalyst precursor, L4 was
synthesized in two steps where in the first one the ligand
L3 was produced. It was initiated by the reaction of a
substituted cyclic amine (2,6-diisopropylaniline) and
diketone (2,6 diacetyl pyridine) (as shown in Scheme 1),
in equimolar amounts, with methanol and formic acid as a
promoter to produce the keto-imine intermediates L3.
Then, in the second step, L4 was synthesized,
initiated by the reaction that occurs between the product of
the first reaction in equimolar amounts and diamine
(p-phenylene diamine).
According to Bianchinni et al. [15], an important
contribution to the selectivity of this reaction was made by
the low solubility of the monoimine products (in
methanol), which precipitated upon formation and so
suppressed further condensation with aniline. However,
the bis(imino) pyridine products were indeed obtained in
low amounts (2–4 %), and so the crude reaction products
were purified by repeated extraction of the (imino)pyridyl
ketones with a boiling ethanol. Once purified, these
products were treated with the appropriate neat primary
amine at 373–383 K, in the presence neither of solvent nor
of acid promoter, to give the expected 2,6-bis(imino)
pyridyl ligands. Under these mild conditions, the
bis(imino) compounds were obtained selectively.
2.3. Synthesis and Characterizations
of Iron(II) Complexes
A suspension of the ligand in n-butanol was added
dropwise at 353 K to a solution of FeCl2 (1.89 mmol) in
dried n-butanol (20 ml) to yield, in general, a blue
solution. After being stirred at 353 K for 15 min, the
reaction was allowed to cool to room temperature. The
obtained material was concentrated, and diethyl ether
(30 ml) was added to precipitate the product as a blue
powder, which was subsequently washed 3 times with diethyl ether (3x10 ml); after this, it was filtered and dried [14].
The analysis of infrared absorption spectrometry
with Fourier transform (FTIR) showed the absence of
absorption band in ~1700 cm-1 of C=O group and the
characteristic band of C=N indicating the formation of the
ligand molecule bis(imino) pyridine at 1620–1640 cm -1.
2.4. Polymerization Experiments
All polymerization reactions were performed in 1 l
Büchi glass reactor equipped with a mechanical stirrer,
connected to a thermostatic system. In a typical
polymerization experiment, the autoclave was charged
with 100 ml toluene and the system was fed with ethylene
up to the desired pressure. The appropriate amount of the
complex and MAO (mixed previously) was introduced at
the temperature of 323 K. The mechanical stirrer was run
with 600 rpm for all experiments to avoid diffusion
controlled polymerization reactions. The molar ratios
between Al/Fe (100, 200 or 2000) and ethylene pressures
in the reaction medium (0.2 or 0.4 MPa) were the only
Synthesis and Evaluation of New Bis(imino) Pyridine Based Catalysts for Ethylene Polymerization 415
parameters varied. Al/Fe was varied by decreasing the
amount of Fe catalyst in the polymerization medium.
After the desired polymerization time (60 min), the
polymerization was stopped by quenching with ethanol
containing 5% HCl. Polymers were dried overnight at
333 K, washed with 5% HCl in an ethanol solution and
after with methanol for several times until total clean of
the supernatant and dried again to a constant weight.
Catalytic activity was determined using Eq. (1):
C. A =
Yield
(molFe) ⋅ ( molE ) ⋅ t
using 4 mg sample, heating from room temperature to
473 K at 10 K/min and cooling at the same rate. The
second heating was performed to measure the melting
temperature. Besides evaluating melting and crystallization temperatures, the degree of crystallinity of the
polymer was also obtained from the melting enthalpy. The
value adopted for the melting enthalpy of polyethylene
100% crystalline was 293 J∙g-1.
Vinylene, vinyl and vinylidene unsaturations are
originated at the end of the polymer chain by transfer
reactions, through Hβ-elimination mechanism, and were
used to determine the number-average molecular weight
(Mn) of the copolymers. Molecular weights of standard
polyethylenes were previously determined by gel
permeation chromatography and their FTIR spectra were
recorded, in order to build a calibration curve. The areas
of vinylene (965 cm-1), vinyl (908 cm-1) and vinylidene
(888 cm-1) groups were calculated, resulting in a total
area, which was normalized in relation to the film
thickness of the sample. The calibration curve was then
employed to determine the number-average molecular
weight of the copolymers [16].
(1)
where Yield – amount of obtained polyethylene, kg;
(molFe) – amount of the catalyst in moles; (molE) –
amount of ethylene in the reaction medium
(0,2 MPa = 0.0135 mol); t – time of polymerization, h.
2.5. Polymer Characterization
The X-ray diffractometer worked with a potential
difference across the tube of 30 kV and 15 mA. The
sweep was carried out in the range of 2θ from 2° to 40°,
while the goniometer speed was 0.05°/min. The radiation
used was CuK of λ = 1.5418 Å.
Thermogravimetric analysis (TA Instruments, TGA
Q500) was performed under nitrogen, heating from room
temperature to 973 K at 10 K/min. Differential scanning
calorimetry (TA Instruments, DSC Q200) was carried out
3. Results and Discussion
In the present work the bis(imino) compounds were
obtained selectively according to Scheme 1.
L1
+ 2 H2O
L2
(a)
+ H2O
L3
(b)
L4
Scheme 1. Synthesis of 2,6-bis[(phenylimino)]pyridine ligands L1 and L2 (a) and
2-(alkylimino)-6-(arylimino)pyridyl ligand–L4 (b)
416
Suellem Cordeiro et al.
Table 1
Structures of the ligands L1-L4
Code
Ligand
CH3
H3C
CH3
CH3
H2N
NH2
H3C
H3C
CH3
CH3
L1
N
N
N
H3C
CH3
2,6–bis[bis(4-phenylimino)-1,4-diisopropylbenzyl]pyridine
N
N
L2
N
H3C
CH3
2,6-bis[(1-phenylimino)ethyl]pyridine
CH3
CH3
O
N
L3
N
H3C
H3C
CH3
CH3
1-{6-[(2,6-Diisopropylphenyl)ethanimidoyl]-2-pyridinyl}-1-ethanone
CH3
NH2
CH3
N
L4
N
N
H3 C
H3 C
CH3
CH3
N-{1-[6-(aminophenyl-ethanimidoyl)-2-pyridinyl]ethylidene}-2,6diisopropylaniline
Table 1 shows the structure of the synthesized
ligands (L1-L4) for subsequent complexation reaction,
except that the ligand L3 that was used only as a precursor
for the synthesis of the ligand L4.
3.1. Characterization of Ligands
and Complexes
The ligands and iron(II) complexes were
characterized by elemental analysis, IR 1H- and 13C-NMR
spectroscopy, as follows.
3.1.1. Ligand L1 and catalyst C1
Yield 5.21 g (55 %). Anal. calc. for C49H45N5 (%):
C, 83.60; H, 6.44; N, 9.94. Found: C, 83.93; H, 6.38; N,
10.04. M.p. 425–426 K. 1H-NMR (300 MHz, CDCl3):
δ(ppm) = 1.68 (s, 12H, CMe2), 1.73 (s, 12H, CMe2),
2.64 (s, 6H, Me-C=N), 6.58 (d, 6.84 Hz, 4H, p-Ar-H),
7.01(d, 6.84 Hz, 16H, C=N-Ar-Ho), 7.27 (d, 6.84 Hz, 4H,
o-ArAniline-H), 7.34 (t, 1H, Py-Hp), 8.12 (d, 2H, Py-Hm).
13
C-NMR (75 MHz, CDCl3): (δ ppm) = 170.15 (Ar-N=C),
159.32 (Ar-NH2), 151.95 (Py-Co), 144.65 (N-Ar-Co),
142.86
(Araniline-Co),
140.85
(Ar-Cm),
128.40
(Araniline-Cm), 127.49 (Ar-Cq), 126.36 (Py-Cm), 125.51
(Py-Cp), 115.33 (N=C-Me), 45.88 (Cq-Me2), 30.33 (CH3C=N), 21.97 (CH3isop).
By FTIR analysis of the ligand L1 and its
respective catalyst (C1), according to Fig. 1 it can be
observed the absence of peak at about 1700 cm-1,
indicating the absence of the carbonyl group, contributing
to synthesis of the ligand.
Synthesis and Evaluation of New Bis(imino) Pyridine Based Catalysts for Ethylene Polymerization 417
It is still possible to observe a shift of the band
characteristic of the C=N bond of the ligand in the
complex, since after the complexation reaction, there is a
shift toward lower wave number (1635–1599 cm-1)
corresponding to the peak of C=N bond, indicating that
the complexation reaction was successfully performed.
3.1.2. Ligand L2 and Catalyst C2
Yield 0.635 g (66 %). Anal. Calc. for C21H19N3 (%):
C, 80.48; H, 6.11; N, 13.40. Found: C, 79.98; H, 6.29;
N, 13.06%. M.p. 431–433 K. 1H-NMR (300 MHz, CDCl3):
δ(ppm) = 2.42 [s, 6H, C(CH3)(CH3)], 7.07 (dd, J = 7.7, 1.2
Hz, 2 H, CH Ar), 7.25-7.30 (m, 4 H, CHAr), 7.32-7.37 (m,
4 H, CHAr), 7.79-7.82 (dd, J = 6.7, 1.2 Hz, 2 H, CH Ar),
7.92 (d, J = 7.7, 1.2 Hz,1 H, CH Ar). 13C-NMR (75 MHz,
CDCl3): δ(ppm) = 167.92 (N=C-Me), 159.33 (Py-Co),
148.36 (CAr-N), 128.73 (Cm-Ar), 123.63 (Cp-Ar),
120.64 (Co-Ar), 125.81 (Py-Cp), 124.68 (Py-Cm), 22.01
(CH3-C=N).
By means of FTIR analysis shown in Fig. 2 the same
behavior (L1/C1) was identified for L2/C2.
a)
3.1.3. Ligand L3
Yield 0.456 g (83 %). Anal.calc. for C21H26N2O (%):
C 78.22, H 8.13, N 8.69. Found: C 78.38, H 8.16, N 8.74.
M.p. 435–437 K. IR: ν(C=N) 1648 cm-1, ν(C=O) 1698 cm-1.
1
H-NMR (300 MHz, CDCl3): δ(ppm) = 1.23 [d,
J = 6.9 Hz, 6H, CH(CH3)(CH3)], 1.25 [d, J = 6.9 Hz, 6 H,
CH(CH3)(CH3)], 2.45 [s, 3 H, C(NAr)CH3], 3.22 [sept,
J = 6.9 Hz, 2 H,CH(CH3)(CH3)], 2.42 [s, 3 H, C(O)CH3],
7.17-7.20 (m, 3 H, CHAr), 7.40 (t, J = 7.8 Hz, 1 H, CH Ar),
7.86 (dd, J = 7.7, 1.2 Hz,1 H, CH Ar), 7.98 (dd, J = 7.9,
1.2 Hz, 1 H, CH Ar). 13C-NMR (75 MHz, CDCl3):
δ (ppm) = 199.8 [1 C, C(O)CH3], 166.34 [1 C,
C(NAr)CH3], 158.72(1 C, C Ar), 146.29 (1 C, C Ar),
137.74 (1 C, CH Ar), 135.69 (2 C, C Ar), 124.92 (1 C, CH
Ar), 124.04 (1 C, CH Ar), 122.97 (2 C, CHAr), 123.2 (1 C,
CH Ar), 28.30 [2 C, CH(CH3)(CH3)], 26.24 [1 C,
C(O)CH3], 23.29 [2 C, CH(CH3)(CH3)], 23.05
[2 C,CH(CH3)(CH3)], 22.85 [1 C, C(NAr)CH3].
All results are according to the literature [15].
b)
Fig. 1. FTIR analysis of the ligand L1 (a) and its respective catalyst C1 (b)
a)
b)
Fig. 2. FTIR analysis of the ligand L2 (a) and its respective catalyst C2 (b)
418
Suellem Cordeiro et al.
a)
b)
Fig. 3. FTIR analysis of the ligand L4 (a) and its respective catalyst C4 (b)
3.1.4. Ligand L4 and catalyst C4
Yield 0.456 g (83 %). Anal. calc. for C27H32N4 (%):
C 79.4, H 7.76, N 13.6. Found: C 78.25, H 7.51, N 14.12.
1
H-NMR (300 MHz, CDCl3): δ(ppm) =1.35 [d,12 H,
(CH(CH3)(CH3)2)], 2.59 [sept, J = 6.9 Hz, 2H,
CH(CH3)(CH3)], 3.40 [s, 3H, C(NAr)CH3], 3.46 [s, 3H,
C(NAr)CH3], 3.89 (s, 4H, NH2), 6.57 (d, 2Hm-ArAniline ),
6.60 (d, 2Ho-ArAniline ), 7.17 (m, 3H, Ar-H), 7.27 (t, 1H,
Py-Hp), 8.10 (d, 2H, Py-Hm). 13C-NMR (75 MHz,
CDCl3): 170.15 (ArAniline-N=C), 169.25 (Ar-N=C), 159.32
(Ar-NH2), 145.63 (Py-Co), 143.59 (Araniline-Co), 142.40
(Araniline-Cm), 140.46 (Ar-Co), 133.20 (Ar-Cp), 125.99
(Py-Cm), 124.77 (Py-Cp), 123.50 (Ar- Cm), 115.03 (N=CMe), 22.94 (CH3-C=N), 30.69 (CHMe2), 21.97 (CH3isop).
FTIR analysis shown in Fig. 3 presents the same
behavior of L1/C1 and L2/C2 for the system L4/C4.
3.2. Results of Ethylene
Polymerization
In this work, three catalysts were evaluated where
the first (C1) contains ligands with a bulky ortho-alkyl
substituent, the second one (C2), existing in the literature,
has the unsubstituted binder and the less bulky moieties;
and finally the third one (C4) has an asymmetric ligand,
which has intermediate bulkiness with only one side of its
face, containing ortho-alkyl substituent.
These differences in a catalyst structure and the
reaction conditions were presented in Table 2, where the
obtained yield of ethylene polymerization and the
catalytic activity using the homogeneous catalysts 1, 2 and
4 are also presented.
For each catalyst studied in homogeneous systems,
polymerizations were performed in order to evaluate the
effect of the Al/Me molar ratio in a catalytic activity.
3.2.1. Influence of ratio [Al]/[Fe]and structure
of the ligands
Regarding the results obtained in polymerizations
using the unprecedented catalyst C1 based on
bis[(phenylimino)]pyridine iron, it can be observed that
the catalytic activity increases with increasing Al/Fe molar
ratio (the ratio between the concentrations of cocatalyst
(MAO) and the catalytic iron complex see Table 2). This
result is consistent with that reported in the literature [14].
It is suggested that, for bulky catalysts larger amounts of
MAO should be required.
Therefore, for the catalyst C1, the presence of a
bulky group substituted in the ortho-position of the
aromatic ring would provide huge steric effect, which
could inhibit the β-hydride elimination and benefit the
propagation of polymer chain. However, as in the
catalysts C2, the rotation of C-N bond is free and it was
observed that there was low contribution to protect the
active site [17].
Polymerizations results with the catalyst C4 at
different Al/metal molar ratios (100, 200, 2000) also show
that the highest activity was achieved in the reaction using
the highest Al/metal ratio (run 9). The same behavior
aforementioned was observed, this is, the catalytic activity
tends to increase with increasing Al /metal ratio in the
reaction medium. The presence of higher amounts of MAO
in the reaction medium provides elimination of impurities
in the polymerization system and better performance of the
cocatalyst as a counterion in these catalysts systems that
contain less bulky groups. Moreover, the structure of C4
has an isopropyl substituent in ortho-position of one aniline
ring, which provided hindrance to termination reactions.
Further studies should be conducted to report the effect of
the structure with NH2 group on the catalytic activity but it
is believed that this group, which contains pairs of free
electrons, may be negatively influencing, for example, the
activity of the catalyst C4.
Synthesis and Evaluation of New Bis(imino) Pyridine Based Catalysts for Ethylene Polymerization 419
Table 2
Polymerizations performed with catalysts 1, 2 and 4 in homogeneous systems
b
Polymer
samples
Fe,
mmol
[Al]/
[Me]a
Yield, g
Catalytic
activity
1
P1 0.1
0.1
100
traces
–
2
P1 0.05
0.05
200
2.390
3541
P1 0.005
0.005
2000
0.258
3822
4
P2 0.1
0.1
100
2.218
1643
5
P2 0.05
0.05
200
1.661
2461
P2 0.005
0.005
2000
traces
–
7
P4 0.1
0.1
100
1.433
1061
8
P4 0.05
0.05
200
0.029
43
P4 0.005
0.005
2000
1.768
26192
Run
3
Structure of catalyst
C1
6
C2
9
C4
Notes: aconcentration of cocatalyst methylaluminoxane is constant and concentrations of metal are 0.1, 0.05 and 0.005 mmol;
catalytic activity: kg/molFe∙molE∙h. Polymerization conditions: 100 ml toluene, cocatalyst methylaluminoxane, 323 K, ethylene
pressure 0.2 MPa.
b
3.2.2. Characterizations of polyethylene by
XRD analysis
3.2.3. Characterizations of polyethylene by
DSC and TGA
It was shown in Fig. 4 the X-ray diffractograms of
the polymer P1-run 2, P2-run5 and P4-run8, where the
presence of peaks at 2θ at about 21 and 24 degrees is
characteristic of the crystal structure of polyethylene,
which is marked as crystallographic planes (110) and
(200), respectively. These planes identify the
orthorhombic unit cells of polyethylene.
It is also possible to observe that the amorphous
halo of polyethylene P1 seems to be higher than for the
polymer P2, which is in agreement with the results of
DSC, which showed higher values of Tm and Tc for P2
(Table 3).
The diffractogram of the polymer P4 has two PE
crystal diffractions, but with lower intensity compared to
P1 and P2 polymers. As the activity of this catalyst was
very low, it is shown that the polymer contains high
amounts of catalyst residue, which is confirmed by the
thermogravimetric analysis also presented in Table 3.
DSC analyses of the polyethylenes were performed
and values of crystallization temperature (Tc), melting
temperature (Tm) and crystallinity degree (Xc) of samples P1
and P2 are shown in Table 3. For P1, the Tm and Tc are
smaller than for the polymer obtained with the catalyst C2
(P2), and this indicates that P1 polyethylene crystals are
smaller and more defective, probably due to difficulties in organizing polymer chains into crystals. This indicates that the
polyethylene obtained by the catalyst C1 is more branched.
The degree of crystallinity of P1 and P2 calculated
based on the melting enthalpy of polyethylene, showed
very similar levels of crystallinity. It was unable to
analyze P4 by DSC due to the low yield obtained.
The thermograms of DSC of polymers P1 and P2
are shown in Fig. 5.
The endotherm of P1 presents a large monomodal
peak, differing from the profile obtained for P2, which is
bimodal, indicating the presence of very different
crystallites sizes.
420
Suellem Cordeiro et al.
Fig. 4. X-ray diffractograms of the polyethylene sample P1 (run 2), P2 (run 5) and P4 (run 8)
Fig. 5. DSC thermogram of the polyethylene sample P1 (run 2), P2 (run 5)
Table 3
Thermal properties of polyethylenes synthesized with catalysts 1, 2 and 4
PE
P1
P2
P4
Tc, K
389
392
–
DSC
Tm, K
399
406
–
Xc, %
44.4
41.8
–
Tonset, K
729
731
699
The TGA profiles of P1, P2 and P4 are
characteristics of the polyethylene decomposition, which
generally occurs from 703 to 773 K. It was possible to
observe that polymer P4 showed significant amounts of
residue (approximately 20 %). This residue is arising from
the presence of catalyst residue and the formation of
aluminum hydroxide by the hydrolysis reaction of the
MAO solution used to precipitate the polymer, which was
obtained in low yield. These residues may have
accelerated the degradation of the polymer resulting in the
decrease at degradation temperatures.
3.2.4. Characterizations of polyethylene by FTIR
The polymers were analyzed by FTIR in order to
observe the presence of vinylene, vinyl and vinylidene
TGA
Tmax, K
751
750
738
Residue, %
1.7
3.6
20.1
FTIR
Mn, g/mol
7.1∙103
7.5∙103
–
end-groups that appear after the active site undergoes
chain transfer reactions.
Polyethylenes P1 and P2 have an average molar
mass (Mn) next to 7,000 g/mol. This value is considered
low and it is possibly caused by the absence of stereohindrance in the ortho position on the aniline moieties of
the catalyst skeleton, which causes the increase in chain
termination reactions forming more terminal groups,
especially vinyl groups, with relatively high absorption in
the region of 908 cm-1, due to β H transfer reaction. As
reported in the literature [18], bulky substituents in the
ortho-position suppress the chain migration process and
also the chain termination reactions.
Catalyst C1 contains sterically bulky ligands,
although the coordination sphere with the metal center is
Synthesis and Evaluation of New Bis(imino) Pyridine Based Catalysts for Ethylene Polymerization 421
not sufficiently protected in order to reduce the transfer
reactions. In this case, the two aromatic rings near the
metal must be perpendicular to the pyridine ring, leaving
the active sites free to side reactions.
4. Conclusions
The
2,6-bis[bis(4-phenylimino)-1,4-diisopropylbenzyl]pyridine and N-{1-[6-(aminophenyl-ethanimidoyl)-2-pyridinyl]ethylidene}-2,6-diisopropylaniline ligands were prepared and reacted with iron chloride to
form the title complexes, sharing the similar framework as
complexes containing 2,6-bis(imino) pyridines. The new
iron complexes exhibited high activities for ethylene
polymerization in the presence of MAO. The higher
activities were at 323 K, and Al/Me molar ratio was
2000 with the pressure of ethylene of 0.2 MPa. Unit cells
of polyethylene were identify as orthorhombic, the
melting temperature was low, 399 K, and the degradation
Tonset was 729 K.
It is useful to highlight the performance of a new
catalyst C1, which has the high catalytic activity in
homogeneous systems.
Acknowledgments
This work was financially supported by CNPq and
CAPES (Brazil).
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СИНТЕЗ І ОЦІНКА НОВИХ КАТАЛІЗАТОРІВ
ПОЛІМЕРИЗАЦІЇ ЕТИЛЕНУ НА ОСНОВІ
БІС(ІМІНО)ПІРИДИНУ
Анотація. З використанням різноманітних лігандів
синтезовані нові трикомпонентні каталітичні системи на
основі 2,6-біс(іміно)піридин хлориду заліза(II), які можуть
бути альтернативними каталізаторами полімеризації
етилену. За допомогою Фур’є-спектроскопії проведено аналіз
синтезованих каталізаторів. Виявлено відсутність смуг
поглинання в області карбонільних груп, і наявність смуг в
області іміногрупи, що відповідає C=N зв’язкам. Встановлено
відповідність з Fe. Структура лігандів і нових каталізаторів
підтверджена за допомогою елементного аналізу, 1H- та 13СЯМР спектроскопії. При полімеризації етилену з метилалюмоксаном як співкаталізатором, встановлено високу активність каталізатора C1. Незважаючи на те, що каталізатор містить стерично об’ємні ліганди, металевий центр
не є достатньо захищеним і тому мають місце реакції
перенесення, внаслідок чого отримується поліетилен з низькою
молекулярною масою і температурою плавлення.
Ключові слова: пост-металоценовий каталізатор,
біс(іміно) піридинові ліганди, полімеризація етилену.